A field chopper, mower, threshing machine, or the like normally has a frame that moves along the ground and that has a front end provided with a device for cutting the crop--corn, alfalfa, grass--at the base. An intake conveyor receives the crop from the cutter and compacts it into a stream which it transports back from the front-end cutter to a blade-carrying drum comminuter which rotates to chop the cut crop. This comminuter or chopper can be seriously damaged if a hard object like a stone, a piece of fencing, or the like is fed by the intake conveyor to it. However, since such a machine is normally being moved through a standing crop that effectively obscures what is directly in front of it, it is very difficult for the operator of the apparatus to see what is being cut and taken in. Thus recourse must be had to automatic detectors.
The detecton of magnetizable or magnetically attractable objects is a relatively simple task. It can be done as described in U.S. Pat. No. 3,757,501, German Pat. No. 2,552,805 (filed by W. Garott with a claim to a priority of Dec. 4, 1974 of U.S. patent application Ser. No. 529,305), and East German Pat. No. 111,534 issued Feb. 20, 1975 to K. Wehsely by making the incoming crop stream pass through a magnetic field. When a potentially dangerous magnetizable object is detected an alarm is issued and/or the intake device and/or comminuter are stopped so that the object can be removed from the machine. The main disadvantage of such a system is that stones or similar solid objects do not have magnetic properties to make them detectable by such equipment. Objects like stones, blocks of wood, bones, or aluminum fencing can do as much harm as magnetizable objects and are just as likely to be encountered when harvesting. In addition when a piece of steel fence wire or the like is in the incoming crop stream it can get wound into the crop stream so is not easily detected by the field, particularly when generated by a plurality of independent coils as in above-cited patent No. 111,534.
Another system is described in East German Pat. Nos. 117,030 and 120,782 both of K. Wehsely. In these arrangements a pusher roller presses the stream of crop down against the conveyor and has at least two relatively displaceable portions. Since the crop is typically fairly soft, at least compared to the hard objects that should be detected by the device, the backwardly flowing cut-crop stream will be crushed and the two portions of the pusher-down roller will not move appreciably relative to each other. When, however, a stone or the like is pinched between one of the roller portions and the conveyor, this portion will be shifted relative to the other portion, typically by pivoting aboout an eccentric but parallel axis. Position-sensing mechanism will detect this relative movement and generate an error signal that will cause the controller to issue a warning and/or stop the machine. Such an arrangement does not work when the object being sensed strikes several portions, as it is relative movement of adjacent portions that is sensed. Furthermore an object imbedded in the crop will frequently also be missed as in most of the prior-art systems.
A further system is described in West German Pat. No. 3,213,713 (filed by W. Raeck with a claim to a priority of Apr. 15, 1981 of U.S. patent application Ser. No. 254,319) where the pusher roller is hollow and is provided internally with transducers which detect the characteristic ping made when a stone or similar hard object strikes this metallic pusher roller. For this system to work this pusher roller must be mounted so that it is insulated from the normal considerable vibration and noise produced during operation of the harvester. In addition the electrical signals produced by the crystal microphones used as transducers must be transmitted through trouble-prone commutator rings to the controller so that these signals can be analyzed and the machine shut down when a hard object is heard striking the pusher roller. Not only is such a device quite complex and, hence, expensive, but it also does not respond when a sufficient layer of soft material comes between the pusher roller and the conveyor, as this masks the sound made by the hard object.